1. Field of the Invention
The invention relates to the field of aeronautics, in particular to aircraft manuevering and control devices of a fixed wing aircraft with a human pilot assisted by an automatic flight control system.
2. Brief Description of the Related Art
Until about 1978, the region beyond stall was considered an unacceptable flight regime frequently characterized by uncontrollable flight in spins and by undesirable deep stalls. Any deep stall condition is characterized by a stable trimmed flight but at a high angle of attack from which return to normal flight may be difficult or impossible. A deep stall may be defined as an out-of-control condition at an angle of attack greater than the angle of attack for maximum lift with no significant motion other than a high rate of descent. Conventional airplanes usually stall and lose control effectiveness at angles of attack in the range of 18.degree. to 20.degree..
However, according to U.S. Pat. Nos. 4,261,533 and 4,099,687, it is now possible, through the use of a rotatable horizontal tail on aft-tail configurations or through the use of tiltable engines on the wings, to provide stable and controllable flight at extremely high airplane angles of attack.
Because movement other than a high rate of descent can be controlled by varying thrust levels and all moveable control surfaces with large deflections, the safety and usefulness of flight at extremely high angles of attack are being re-examined and redefined.
The essence of the longitudinal control concept, as set forth in U.S. Pat. Nos. 4,261,533 and 4,099,687, is to rotate the tail or to deflect large chord elevons to magnitudes of approximately the same order, but of opposite direction, as the airplane angle of attack, so that the effective tail aerodynamic angle of attack is below the tail stall angle and is thus capable of providing both stability and control for the entire aircraft.
Although rotatable canard arrangements are known from U.S. Pat. No. 4,569,493, U.S. Pat. No. 4,281,810, U.S. Pat. No. 4,010,920, and West German Offenlegungsschrift 2421524, such arrangements deal strictly with the stability and control of aircraft and models in level and unstalled low angle of attack regions of flight and do not address the problems of stability and control of aircraft in the high angle of attack regions of flight.
In most cases, the upper limit of normal flight is associated with conditions for maximum lift, beyond which the wing is completely stalled. For some aircraft configurations, however, for example, those employing wings with high leading edge sweep angles or incorporating strakes, i.e. a continuous band of plates on the fuselage, partial flow separation of the wing or control surfaces may induce stability problems below the attack angles for maximum lift and impose lower limits on the normal flight regions.
Solutions of these problems will allow flight above these lower limits. Flight above the normal limits is considered of a supernormal nature.